The method of electrophotographic development is a method in which toner particles in a developer are made to adhere on an electrostatic latent image formed on a photoreceptor to develop the image. The developer used in this method is classified into a two-component developer composed of toner particles and carrier particles, and a one-component developer using toner particles alone.
As a development method using a two-component developer composed of toner particles and carrier particles among those developers, a cascade method and the like were formerly employed, but a magnetic brush method using a magnet roll is now in the mainstream.
In a two-component developer, a carrier particle is a carrier substance which is stirred with a toner particle in a development box filled with the developer to thereby impart a desired charge to the toner particle, and further transports the charged toner particle to a surface of a photoreceptor to thereby form a toner image on the photoreceptor. The carrier particle remaining on a development roll holding a magnet is again returned from the development roll to the development box, mixed and stirred with a fresh toner particle, and used repeatedly in a definite period.
In a two-component developer, unlike a one-component developer, a carrier particle has functions of being mixed and stirred with a toner particle to charge the toner particle and transporting the toner particle, and has good controllability on designing a developer. Therefore, the two-component developer is suitable for full-color development apparatuses requiring a high image quality, high-speed printing apparatuses requiring reliability and durability in image maintenance, and other apparatuses.
In a two-component developer thus used, it is needed that image characteristics, such as image density, fogging, white spots, gradation and resolving power, exhibit predetermined values from the initial stage, and additionally these characteristics do not vary and are stably maintained during endurance printing. In order to stably maintain these characteristics, properties of a carrier particle contained in a two-component developer need to be stable.
As a carrier particle forming a two-component developer, an iron powder carrier, such as an iron powder whose surface is covered with an oxide film, or an iron powder whose surface is coated with a resin, has conventionally been used. Since such an iron powder carrier has a high magnetization and a high conductivity as well, the carrier has an advantage of easily providing images well reproduced on the solid portion.
However, since such an iron powder carrier has a heavy true specific gravity of about 7.8 and a too high magnetization, stirring and mixing thereof with a toner particle in a development box is liable to generate fusion, so-called toner spent, of toner-constituting components onto the iron powder carrier surface. The generation of such toner spent decreases the effective carrier surface area, and is liable to reduce the triboelectric chargeability with the toner particle.
In a resin-coated iron powder carrier, the resin on the surface exfoliates due to stresses during endurance printing, and a core material (iron powder) having a high conductivity and a low dielectric breakdown voltage is thereby exposed, and leakage of the charge thereby occurs in some cases. Due to such leakage of the charge, an electrostatic latent image formed on a photoreceptor is broken, brush streaks and the like are generated on the solid portion, and a uniform image can hardly be obtained. For these reasons, iron powder carriers such as an oxide film-coated iron powder or a resin-coated iron powder have not been used recently.
In recent years, in place of iron powder carriers, a ferrite having a light true specific gravity of about 5.0 and a low magnetization is mainly used as a carrier, and a resin-coated ferrite carrier whose surface is coated with a resin is often used, whereby the life of the developer has been remarkably prolonged.
The method for manufacturing such a ferrite carrier generally involves mixing ferrite carrier raw materials in predetermined amounts, and thereafter calcining, pulverizing and granulating and then sintering the mixed material; and depending on conditions, the calcination may be omitted.
Meanwhile, the environmental regulation has recently become strict, and the use of metals such as Ni, Cu and Zn comes to be avoided and the use of metals adapted to the environmental regulation is demanded; then, ferrite compositions used as a carrier core material have been shifted from Cu—Zn ferrites and Ni—Zn ferrites to manganese ferrites, Mn—Mg—Sr ferrites and the like, which use Mn.
Patent Literature 1 (Japanese Patent Laid-Open No. 08-22150) describes a ferrite carrier in which a part of a manganese-magnesium ferrite is replaced by SrO. It is contended that when the ferrite carrier is used as a developer with a toner, by reducing a variation in magnetization among ferrite carrier particles, the developer exhibits excellences in image quality and durability, is friendly to the environment, has a prolonged life, and is excellent in environmental stability. However, the ferrite carrier described in Patent Literature 1 cannot satisfy simultaneously both a uniform surface property having a reasonable unevenness and a high charging capability. If the sintering temperature is made high, since the surface property exhibits much of smooth portions and becomes nonuniform, not only the distributions of the resistivity and the charge after resin coating are broadened, but also the strength to stirring stresses decreases. If the sintering temperature is made low, the surface apparently has a wrinkly uniform surface property, but since the value of the BET specific surface area becomes large, the charging property becomes low and the influence by environment becomes large.
Patent Literature 2 (Japanese Patent Laid-Open No. 2004-004648) describes a ferrite carrier having a spinel structure and having a volume-average particle diameter of 20 to 45 μm, which is a resin-coated carrier whose magnetization and surface uniformity are prescribed. In the core material particle described in Patent Literature 2, a high sintering temperature is used; the surface is smooth; and the coating resin hardly infiltrates; and the coating resin easily exfoliates; the achievement of the long life of the carrier is therefore difficult.
Carrier core materials using Mg are proposed as replacements of carrier core materials using Mn. For example, Patent Literature 3 (Japanese Patent Laid-Open No. 2010-39368) describes a carrier core material containing magnesium, titanium and iron in definite proportions and having a BET specific surface area in a specific range. It is contended that the carrier core material provides a desired resistivity, a medium one or a high one, while exhibiting a high magnetization, and is excellent in the charging property, and has both of the surface property having a reasonable unevenness, and uniform shapes.
Since the carrier core material described in Patent Literature 3 has low contents of manganese and titanium, the material basically exhibits properties of a magnetite, and since the magnetization of a low magnetic field side is low, the occurrence of carrier beads carry over in image formation by an actual machine is apprehended.
There is Patent Literature 4 (Japanese Patent Laid-Open No. 2008-96977) in which since the particle diameter reduction of carriers has advanced in recent years along with the particle diameter reduction of toners involved in the image quality enhancement, the dispersion in particle shapes, which has not conventionally become a problem, is studied, and exfoliation of a resin, and uniform resin coating are studied.
Patent Literature 4 discloses a carrier prepared by coating the surface of a core particle composed of a ferrite containing at least a magnesium element, with a resin, and contends that the irregular shape factor of the core particle is 5% by number or less, and the grain diameter of the surface is 2 to 5 μm. Patent Literature 4 contends that the use of such a core particle imparts a sufficient charging property to a toner, and provides a stable charging property never causing image contamination such as fogging caused by toner scattering due to charge insufficiency.
However, Patent Literature 4 defines the shape of a core material particle only by an irregular shape factor of the core, and only pays attention especially to portions whose shape is remarkably poor, which is not enough in order to evaluate the dispersion of the particle shape as a whole, and to substitutionally evaluate carrier scattering and the like. Additionally, Patent Literature 4 carries out the improvement of the environmental dependency of a carrier only by resin coating. Therefore, even if the environmental dependency of the carrier right after the usage start in actual usage is good, a coating resin exfoliates as the usage time is prolonged, and the surface of the core particle is exposed to thereby lose the environmental dependency gradually; therefore, Patent Literature 4 is not enough from the viewpoint of the improvement of the environmental dependency.
Patent Literature 5 (Japanese Patent Laid-Open No. 2007-271662) describes a resin-coated ferrite carrier in which the apparent density, the average particle diameter and the BET specific surface area of a carrier core material have a definite relation. Since the core material particle described in Patent Literature 5 contains no Sr, not only unevenness is not formed, or hardly formed on the surface depending on the sintering temperature, but the addition of Sr cannot have an increasing effect on the charge of the core material.
In consideration of these conventional technologies, a ferrite carrier for an electrophotographic developer has been demanded which has the uniform particle shape and unevenness of the surface in spite of having a small particle diameter, has reasonable resistivity and magnetization, has an excellent charging property, and hardly causes carrier scattering.